1. Field of the Invention
This invention relates to an image read-out method for reading out an image from a material-to-be-scanned by scanning the material-to-be-scanned carrying the image recorded thereon by a light beam and photoelectrically detecting light obtained from the material-to-be-scanned and carrying the image.
2. Description of the Prior Art
There have heretofore been used widely image readout apparatuses for obtaining image signals by conveying a material-to-be-scanned carrying an image recorded thereon in a sub-scanning direction, scanning the material-to-be scanned by a light beam deflected by a light deflector in a main scanning direction approximately normal to the subscanning direction, thereby obtaining light carrying the image, and photoelectrically detecting the light carrying the image by a light detection means.
By way of example, various laser beam sources can be used as the source of the light beam. As the light deflector for deflecting the light beam to have the light beam scan in the main scanning direction, a rotating polygon mirror rotated quickly is mainly used by virtue of comparatively high scanning stability.
In the case where the rotating polygon mirror is used as the light deflector, the problem as described below arises even though the scanning stability is comparatively high. Specifically, in the case where surfaces of the rotating polygon mirror have even a slight inclination relative to one another (surface inclination, i.e. fluctuations in the angle between a rotation shaft of the rotating polygon mirror and a normal line of each surface), the scanning position of the light beam reflected by the rotating polygon mirror o the material-to-be-scanned fluctuates in the sub-scanning direction among the surfaces of the rotating polygon mirror. As a result, the image signals detected photoelectrically become as if the conveyance in the sub-scanning direction were uneven periodically with a period equal to the time corresponding to a single turn of the rotating polygon mirror.
In order to eliminate the adverse effects of the surface inclination of the rotating polygon mirror, a compensating optical system constituted by a cylindrical lens, a toroidal lens or the like have heretofore been used so that a scanning line is formed at a predetermined position even though surface inclination arises. Such a compensating optical system is large and complicated, and makes the apparatus large and expensive.